1. Field of the Invention
This invention relates to semiconductor fabrication technology, and more particularly, to a method of forming an opening in a dielectric layer through a photoresist layer with silylated sidewall spacers, which can help reduce the critical dimension of the resulting opening.
2. Description of Related Art
High integration is the primary goal in the fabrication of integrated circuit (IC) devices. To achieve this goal, the various components in the integrated circuit are made with the smallest possible dimensions. Current semiconductor fabrication technology is able to fabricate integrated circuits clown to the 0.25 .mu.m (micrometer) submicron level of integration. The photolithography is the key technology to the fabrication of MOS (metal-oxide semiconductor) related semiconductor structures, such as doped areas and contact openings. In submicron integration, the photolithographic transfer of a pattern from a mask is highly critical. Various methods, such as Optical Proximity Correction (OPC) and Phase Shift Mask (PSM), have been proposed to achieve high definition for the pattern transfer from a mask through photolithography onto a photoresist layer.
The OPC method is designed for the purpose of minimizing the deviation in the critical dimension of the transferred pattern due to a proximity effect. During the photolithographic process, the exposure light passing through the mask and striking on the photoresist layer is widely scattered due to dispersion.
Moreover, the exposure light transmitted through the photoresist layer reflects back from the substrate, thus causing interference that then causes double-exposure on the photoresist layer. As a result, the critical dimension of the transferred pattern from the mask is degraded. This undesired proximity effect is illustratively depicted in FIGS. 1 and 2.
In FIG. 1, the dashed rectangular area indicated by the reference numeral 10 is the intended pattern that is to be transferred from the mask onto the photoresist layer. Due to the proximity effect, however, the actually resulting pattern is shrunk in size to the shaded area indicated by the reference numeral 11. In FIG. 2, the dashed areas indicated by the reference numeral 20 are the intended patterns that are to be transferred from the mask onto the photoresist layer. Due to the proximity effect, however, the actually resulting pattern deviates from the intended positions and is inaccurately dimensioned, as are the shaded areas indicated by the reference numeral 21.
The OPC method solves the foregoing problem by first using computer software to compute the dimensional and positional deviations between the resulting patterns on the photoresist layer and the predefined patterns on the mask, and then using the data for correction of the size and position of the patterns on the mask. This increases the accuracy of transfer pattern definition. One drawback to the OPC method, however, is that it requires complex computing to obtain the needed corrections to the mask patterns and is therefore very difficult to implement.
FIG. 3A is a schematic diagram used to explain the principle of photolithography, whereas FIG. 3B is a schematic diagram used to explain the principle of the PSM method. These two diagrams are juxtaposed for the purpose of comparison. As shown in FIG. 3A, conventional photolithography utilizes a mask including a predefined pattern of chromium layers 100 coated over a crystal sheet 150. The chromium layers 100 represent the pattern that is to be transferred onto the photoresist layer (not shown). The chromium layers 100 are nontransparent to light. During the exposure process, exposure light 170 passes through the mask to illuminate the unmasked portions of the photoresist layer (not shown). The amplitude distribution of the exposure light 170 over the mask, the amplitude distribution of the exposure light 170 over the photoresist layer, and the intensity distribution of the exposure light 170 over the photoresist layer are respectively illustrated in the three graphs beneath.
As shown in FIG. 3B, by the PSM method, the mask is additionally provided with a phase-shifter layer 120 which can invert the phase of the light passing therethrough. The light passing through the phase-shifter layer 120 thus interferes in a destructive manner with the neighboring light that has not passed through the phase-shifter layer 120, thus making the transferred pattern sharply defined. The PSM method has the benefit of enhancing the definition of the transferred pattern without having to use shortwavelength exposure light. One drawback to the PSM method, however, is that the mask is quite difficult to manufacture and modify.
Although the foregoing OPC and PSM methods can help enhance the pattern definition in downsized fabrication of integrated circuits, they are still unsatisfactory to use due to complex and costly implementation.